This invention relates generally to method and means to increase the performance of pump systems, at low flow rates, and more particularly concerns means to increase the stable operating range of a pump to near zero flow rate, and to reduce the fluid temperature rise at low flow operation.
Pump diffusers and inducers are well known devices. In particular, a rotating diffuser rotating in the same direction as the impeller uses the shear forces inside and outside of the rotating diffuser structure to establish an equilibrium speed (anywhere between 20% to 40% of the impeller speed, depending on the diameter ratio of the rotating diffuser). In this way the shear forces at the inside of the rotating diffuser are reduced so that the efficiency of the overall arrangement (impeller and rotating diffuser) is improved, when compared to a conventional arrangement (impeller and stationary diffuser).
Additional improvements are obtained when the rotating diffuser is coupled mechanically with a rotating pre-inducer arranged in front of the centrifugal impeller. This inducer-diffuser combination, referred to as the "free spool", typically rotates at a fraction of the main impeller speed, and is particularly beneficial for pumps, since the inducer section of the free spool reduces the cavitation head requirements (NPSH) of the system, so that a higher suction specific speed design results. However, in certain cases, the equilibrium speed of the free spool, resulting from the shear forces acting on the rotating diffuser section, is either too low or too high to obtain the high suction specific speed goal.